Ositus

#include <iostream>
#include <math.h>
#include <vector>
#include <algorithm>
using namespace std;

const int charCount = 26;
int toInt(char c) {
    return (int) c - 97;
}

long all(long len) {
    return pow(2, max(len - 1, (long) 0));
}

long solve(string value) {
    long res = 1;
    long history[value.length()];

    int chars [charCount];
    fill_n(chars, charCount, -1);

    for (long i = 0; (unsigned) i < value.length(); i++) {

        int c = toInt(value[i]);
        if (chars[c] != -1) {
            long start = history[chars[c]];
            long end = all(i - chars[c]) - 1;

            res += start * end;
        } else {
            res *= 2;
        }

        history[i] = res;
        chars[c] = i;
    }

    return res / 2;
}

int main() {
    string str;
    cin >> str;
    cout << (solve(str) % (long) (pow((long) 10, (long) 9) + 7)) << "\n";
}

Test 1

Group: 1, 2, 3

Verdict: ACCEPTED

Test 2

Group: 1, 2, 3

Verdict: ACCEPTED

Test 3

Group: 1, 2, 3

Verdict: WRONG ANSWER

Test 4

Group: 1, 2, 3

Verdict: WRONG ANSWER

Test 5

Group: 2, 3

Verdict: WRONG ANSWER

Test 6

Group: 3

Verdict: WRONG ANSWER

Test 7

Group: 3

Verdict: WRONG ANSWER